SPD light valves have been known for more than seventy years for use in the modulation of light. Such light valves have been utilized in numerous applications during that time including, e.g., alphanumeric displays and television displays; filters for lamps, cameras, displays and optical fibers; and windows, sunroofs, toys, sunvisors, eyeglasses, goggles, mirrors, light pipes and the like to control the amount of light passing therethrough or reflected therefrom as the case may be. Examples of windows include, without limitation, architectural windows for commercial buildings, greenhouses and residences, windows, visors and sunroofs for automotive vehicles, boats, trains, planes and spacecraft, windows for doors including peepholes, and windows for appliances such as ovens and refrigerators including compartments thereof.
As used herein, the term “light valve” describes a cell formed of two walls that are spaced apart by a small distance, with at least one wall being transparent. The walls have electrodes thereon, usually in the form of transparent, electrically conductive coatings. Optionally the electrodes on the walls may have thin transparent dielectric overcoatings thereon. The cell contains a light-modulating element (sometimes herein referred to as an activatable material), which may, without limitation, be a liquid suspension of particles. Alternately, all or a portion of the entire element may comprise a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as a light valve suspension or as a liquid light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement. Hence, a beam of light passing into the cell is reflected, transmitted or absorbed depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state.
For many applications it is preferable for all or part of the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of light suspension, can be avoided through use of a film and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A light valve film (also sometimes herein referred to as an SPD film) as used herein means a film or sheet, or more than one thereof, comprising a suspension of particles used or intended for use in an SPD light valve. Such light valve film usually comprises a discontinuous uncrosslinked phase of a liquid comprising dispersed particles, termed a liquid light valve suspension, such discontinuous phase being dispersed throughout a cured continuous phase enclosed within one or more rigid or flexible solid films or sheets. Cured emulsion, which forms part of a light valve film, is sometimes also referred to as a film or film layer. The light valve film and/or a laminate of the light valve film may also comprise one or more additional layers such as, without limitation, a film, coating or sheet or combination thereof, which may provide the light valve film with one or more of, for example, (1) scratch resistance, (2) protection from ultraviolet radiation, (3) reflection of infrared energy, (4) electrical conductivity for transmitting an applied electric or magnetic field to the activatable material, (5) dielectric overcoatings, color tinting and acoustic control.
A common (but non-limiting) construction for an SPD film may comprise, for example, five layers, namely, in order from a first side to a second, opposed side: (1) a first sheet of polyethylene terephthalate (“PET”) plastic, conveniently 5-7 mils in thickness, (2) a very thin transparent, electrically conductive coating of indium tin oxide (“ITO”), acting or capable of acting as an electrode, on said first sheet of PET, (3) a layer of cured (i.e., cross-linked) SPD emulsion, usually 2-5 mils in thickness and, (4) a second ITO coating acting or capable of acting as an electrode on (5) a second PET plastic substrate. As stated above, additional layers, which provide other functions, may optionally be added to the five-layer SPD film described above. Typically, copper foil, conductive fabric or the like are affixed to the electrodes so that they extend beyond the perimeter of the SPD film for convenient connection to a suitable voltage source. Furthermore the SPD film can be laminated, for example, with transparent hot melt adhesive interlayer films and/or glass or thicker transparent plastic sheets to provide strength and rigidity and to protect various parts of the combined unit from environmental stresses which may, otherwise, damage its performance characteristics.
U.S. Pat. No. 5,409,734 exemplifies a type of non-cross-linked light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking (curing) of emulsions are also known. The methods of the present invention are specifically directed to the use of the latter type of film, i.e., film comprising a layer formed by cross-linking an emulsion, and to laminated films produced therewith. See, for example, U.S. Pat. Nos. 5,463,491 and 5,463,492, and U.S. Pat. No. 7,361,252, all of which are assigned to the assignee of the present invention. Various types of SPD emulsions, and methods of curing same, are described in U.S. Pat. Nos. 6,301,040, 6,416,827, and 6,900,923 B2, all of which are assigned to the assignee of the present invention.
A non-limiting example of such a film from Example 5 of U.S. Pat. No. 6,900,923 B2 follows: 0.002 g of Irgacure 819 (Ciba Specialty Chemicals) photoinitiator (“PI”) was dissolved in 2 mL of chloroform and added to 1 g of the matrix polymer described in Example 1. The PI solution was thoroughly mixed with the matrix polymer and the solvent chloroform was removed by placing the mixture inside of a vacuum oven for 30 minutes at 60° C. To this was added 0.62 g of polyiodide crystal paste containing the lauryl methacrylate/HEMA suspending polymer (0.56 g, as synthesized in example 3). The resulting mixture was thoroughly mixed and the emulsion obtained was applied onto a conductive coated polyester substrate as a 2 mil thick coating using a doctor blade, mated with a blank conductive coated polyester substrate and cured with ultraviolet radiation (8600 mJ/cm2/min) for 2 min and 30 seconds.
Such films and variations thereof may be cured through cross-linking brought about by exposing the films to (1) ultraviolet radiation, (2) electron beams or (3) heat.
All of the patents and patent applications and other references cited in this application are incorporated herein by reference.
Further to the above, a variety of liquid light valve suspensions are well known in the art and such suspensions are readily formulated according to techniques well known to one of ordinary skill therein. The term liquid light valve suspension, as noted above, when used herein means a liquid suspending medium in which a plurality of small particles are dispersed. The liquid suspending medium comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer that acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
Liquid light valve suspensions useful in the present invention may include any of the so-called prior art liquid suspending media previously proposed for use in light valves for suspending the particles, which are well-known to those working in this field. Liquid suspending media known in the art which are useful herein include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175, 4,407,565, 4,772,103, 5,409,734, 5,461,506, 5,463,492, and 6,936,193 B2, the disclosures of which are incorporated herein by reference. In general one or both of the suspending medium or a polymeric stabilizer typically dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid(s) which comprise the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, when dissolved provides a plain surface coating for the particles, together with one or more additional types of solid polymeric stabilizer that when dissolved, bond to or associate with the first type of solid polymeric stabilizer and also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described for example in U.S. Pat. No. 5,463,492.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light absorbing or light reflecting in the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed particles of colloidal size. As used herein the term colloidal means that the particles generally have a largest dimension averaging 1 micron or less. Preferably, most polyhalide or non-polyhalide types of particles used or intended for use in an SPD light valve suspension will have a largest dimension which averages 0.3 micron or less and more preferably averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms, to keep light scatter extremely low.
Prior art SPD films and laminates incorporating such SPD film(s) can develop a pattern at the perimeter of the SPD film where a lighter blue area or “light frame” is visible. The light frame develops when the SPD film or a laminate of the SPD film is exposed to heat and/or moisture.
As mentioned above, prolonged exposure of SPD film or laminates of SPD film to elevated temperatures and/or humidity can cause a light frame to appear. U.S. Pat. No. 7,361,252, assigned to the assignee of the present invention, describes the use of adhesive tape to seal the edges of the SPD film to prevent the hot melt adhesive interlayer material from coming into contact with the cured SPD emulsion. However, the tape in that instance was used as a physical barrier to the interlayer material and did not possess moisture resistant properties.
Another deficiency of prior art SPD films was the tendency of the cured emulsion to absorb moisture through the exposed edge of the SPD film over time if measures were not taken to isolate the SPD film from sources of such moisture. Subsequent exposure of the SPD film or laminates of the SPD film to elevated temperatures and/or humidity could lead to a defect such as the light frame mentioned above.
PCT/GB2010/051566 describes drying the interlayer plies to a predetermined moisture content to reduce or eliminate degradation of an SPD. However, this application does not consider the moisture content in the SPD film itself, moisture-resistant barrier materials to prevent moisture from contacting the SPD film or moisture-resistant border materials to prevent moisture from entering the SPD laminate.
The inventors have, therefore, undertaken to develop a methodology for making light valves of the type described herein wherein the occurrence of undesirable light frames is eliminated or at least substantially reduced.